1. Field of the Invention
The present invention relates to a method for differentiating dynamic range of image, and more particularly to method for differentiating density units in a dynamic range of image.
2. Description of the Related Art
Optical scanners are used to capture and digitize images. For example, an optical scanner can be used to capture the image of printed matter on a sheet of paper. The digitized image can then be electronically stored and/or processed with character recognition software to produce ASCII text. The typical optical scanner includes a light source, a linear array of photoelectric sensing elements (generally a CCD sensor or a CMOS sensor, or a CIS sensor), an analog amplifier, an analog to digital converter (ADC), a controller and a random access memory (RAM).
The CCD sensor includes a large number (e.g., 2000) of photoelectric sensing elements arranged in a linear array. Each photoelectric sensing element will capture light representing a single pixel of the image. The array will capture a line of pixels. By moving the CCD sensor across a document, the entire document can be scanned one line at a time.
The conversion into digital signals of light reflected from or transmitted through the document takes place in essentially three steps. First, each photoelectric sensing element will convert the light which it receives into an electric charge. The magnitude of the charge will depend on the intensity of the light and the exposure time. Second, the charges from each of the photoelectric sensing elements are converted into analog voltages via the analog amplifier. Finally, the analog voltages are digitized by the analog to digital converter for digital image processing and storage in the RAM.
Images of a target or a test chart finally generated by a scanner have density units with separate pixel-level distribution. Although density units have individual pixel-level distribution, the pixel-level distributions of density units could overlap so as to render the density units undistinguishable. Conventionally, density units Dn and Dn+1 are differentiated by a standard deviation method, wherein when Dn−1/4Sn>Dn+1+1/4Sn+1, density units Dn and Dn+1 are distinguishable. However, the standard deviation method still has several disadvantages. For example, when some obstacles or dusts are present on a target or a test chart, the images of the obstacles or dusts would interfere the differentiation of density units by the standard deviation method.
Hence it is desirable to provide a method for differentiating dynamic range of image to overcome the disadvantages of the conventional method.